The successful development of the tilt-rotor aircraft in the last 30 years (Bell XV-15, Bell/Boeing V-22 and Bell/Agusta 609) make the tilt-rotor configuration a commercially viable starting point for efficient Vertical Take-Off and Landing (VTOL) aircraft.
Tilt-rotor aircraft are aircraft which use the lift of rotors to hover and VTOL. These aircraft tilt their rotors so that in forward flight the lift is provided by the wing and the forward thrust by the rotors. The term “proprotor” can also be used to describe rotors that function as a propulsive device in forward flight and as lifting device in helicopter mode.
The cruise efficiency of aircraft as measured by its payload carried times the distance traveled per consumed fuel (for example Lb of Payload×Mile traveled/Lb of consumed fuel) is proportional to the ratio between lift and drag of the aircraft in cruise.
The best (highest) lift/drag ratio of a fixed wing aircraft is strongly related to the ratio of wing span to the size of the aircraft. For example, competition gliders use very small and streamlined fuselage (for low drag) and large span wings for best lift/drag (glide ratio).
The flight speed for best lift/drag ratio, at given aircraft weight and altitude, is a function of wing area. An aircraft with smaller wing area will have higher speed for best lift/drag. The ratio of wing span to average wing chord is called wing aspect ratio. The combination of increasing glide ratio (larger span) and decreasing wing area (increasing speed) result in a strong drive to increase the wing aspect ratio (long and narrow wings). High wing aspect ratios are limited by structures, weight and structural dynamics considerations.
While high performance gliders use wing aspect ratio ranging from 20 to 38, the values for modern swept back wings of jet transports are 8-10 and for straight wings of propeller driven transports are 10-12. The use of high strength/weight carbon fiber composites makes higher aspect ratio wings more efficient in terms of aerodynamic performance vs. wing weight.
Prior art tilt-rotor aircraft have wing aspect ratio of 5.5 with the tilt-rotors, engines and nacelles placed at the wing tips. A particularly important consideration for such a low aspect ratio having a very stiff wing to avoid whirl flutter, which is an aero-elastic instability of the combination of wing and rotor. The wider chord wing of 5.5 aspect ratio causes a high down-load in hover of 11-12% of rotor lift therefore requiring larger rotors, more powerful engines and higher torque gearboxes to overcome the increase in required rotor lift.
All current tilt-rotor aircraft have adopted the same sense of rotor rotation, top blade turning outward. This sense of rotation provides an interaction between the rotor and wing which is equivalent to approximately 10% increase in wing aspect ratio.
Tilt-wing aircraft (as opposed to tilt-rotor aircraft) use the lift of rotors or propellers to hover and VTOL with their wings pointed in the vertical position (leading edge up), and tilt their wings and rotors or propellers so that in forward flight the lift is provided by the wing and forward thrust by the rotors or propellers. An example of a tilt-wing aircraft is the LTV/Hiller/Ryan XC-142A. Among the disadvantages of tilt-wing aircraft is the high level of buffeting caused by the massive stall of the wing in forward flight with the wing and rotors at steep angle close to hover position. This maneuver is necessary for conversion from hover to forward flight at the minimum speed required to generate adequate lift on the wing to allow tilting the wing and rotor to normal airplane flight. Tilt-rotor aircraft don't have that disadvantage because they don't tilt the wing.
A remaining problem in tilt-rotor aircraft is that the low aspect ratio results in low lift/drag ratio, and consequently low aircraft efficiency and fuel economy. Thus, there is still a need for improvements to tilt-rotor aircraft that would improve lift/drag ratio, aircraft efficiency, and fuel economy